High quality digital cameras have to a large extent replaced film cameras. However, like film cameras, with digital cameras much attention has been placed by the camera industry on the size and quality of lenses which are used on the camera. Individuals seeking to take quality photographs are often encouraged to invest in large bulky and often costly lenses for a variety of reasons. Among the reasons for using large aperture lenses is their ability to capture a large amount of light in a given time period as compared to smaller aperture lenses. Telephoto lenses tend to be large not only because of their large apertures but also because of their long focal lengths. Generally, the longer the focal length, the larger the lens. A long focal length gives the photographer the ability to take pictures from far away.
In the quest for high quality photos, the amount of light which can be captured is often important to the final image quality. Having a large aperture lens allows a large amount of light to be captured allowing for shorter exposure times than would be required to capture the same amount of light using a small lens. The use of short exposure times can reduce blurriness especially with regard to images with motion. The ability to capture large amounts of light can also facilitate the taking of quality images even in low light conditions. In addition, using a large aperture lens makes it possible to have artistic effects such as small depth of field for portrait photography.
While large lenses have many advantages with regard to the ability to capture relatively large amounts of light compared to smaller lenses, they can be used to support large zoom ranges which may be implemented using optical or digital techniques, and often allow for good control over focus, there are many disadvantages to using large lenses.
Large lenses tend to be heavy requiring relatively strong and often large support structures to keep the various lenses of a camera assembly in alignment. The heavy weight of large lenses makes cameras with such lenses difficult and bulky to transport. Furthermore, cameras with large lenses often need a tripod or other support to be used for extended periods of time given that the sheer weight of a camera with a large lens can become tiresome for an individual to hold in a short amount of time.
In addition to weight and size drawbacks, large lenses also have the disadvantage of being costly. This is because of, among other things, the difficultly in manufacturing large high quality optics and packaging them in a manner in which they will maintain proper alignment over a period of time which may reflect the many years of use a camera lenses is expected to provide.
In digital cameras, the photosensitive electronics used as the sensor, e.g., light sensing device, is often either a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) image sensor, comprising a large number of single sensor elements, each of which records a measured intensity level.
In many digital cameras, the sensor array is covered with a patterned color filter mosaic having red, green, and blue regions in an arrangement. A Bayer filter mosaic is one well known color filter array (CFA) for arranging RGB color filters on a square grid of photo sensors. Its particular arrangement of color filters is used in many digital image sensors. In such a filter based approach to capturing a color image, each sensor element can record the intensity of a single primary color of light. The camera then will normally interpolate the color information of neighboring sensor elements, through a process sometimes called demosaicing, to create the final image. The sensor elements in a sensor array using a color filter are often called “pixels”, even though they only record 1 channel (only red, or green, or blue) of the final color image due to the filter used over the sensor element.
In cameras, round or circular lenses (lens element) through which light can pass, e.g., lenses with round apertures, are commonly used. This allows light to pass through the lens equaling in both vertical and horizontal directions (actually any direction). Elements in a optical chain in which one or more round lenses are used are often of sufficient size to pass enough light to record a sharp image at the sensor of the camera. For a lens, the optical axis is the line passing through the center of the lens and perpendicular to the plane of the lens. When a lens assembly is constructed out of more than one lens element, the elements are typically arranged so that they all share a common optical axis which is also the optical axis of the lens assembly. In a typical optical chain, e.g., camera module, the optical axis also passes through the center of the sensor. Light traveling along the optical axis of a lens assembly or camera module is not bent by any lens along the path and continues to travel along the optical axis in a straight line. If all the lens elements are circular, then such a camera module has cylindrically symmetry around the optical axis. In most cameras, the optical elements of a camera are arranged in a linear configuration with optical axis passing through an outer lens in a straight line to the sensor. Such a configuration can result in relatively thick cameras, e.g., cameras having a large front to back distance or depth. In cameras with large optics and/or which support mechanical zoom, the camera thickness can be significant with the camera often being several inches thick and far too deep to store in a pocket or even in some cases a purse.
Cameras and other optical systems are often discussed in terms of focal length. The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus. A system with a shorter focal length has greater optical power than a system with a long focal length; that is, it bends the rays more strongly, bringing them to a focus in a shorter distance. Longer focal length (lower optical power), often achieved using large lenses, leads to higher magnification, e.g., zoom, and a narrower angle (field) of view. Accordingly, an optical chain, e.g., camera module, with a large, e.g., long, focal length will capture an image corresponding to a smaller portion of a scene area than an optical chain at the same location with a smaller focal length. It should be appreciated that for the same sensor size, an optical chain with a shorter focal length or higher optical power is associated with a wider angle of view than an optical chain with a longer focal length and will thus capture an image corresponding to a larger portion of a scene area at the same distance from the optical chain than an optical chain at the same position with a larger focal length.
Focal length of a lens element is generally not a function of lens size, e.g., diameter in the case of lens element lenses with round apertures, e.g., round areas through which light can pass. In an optical chain or other optical device the focal length of the device, is sometimes referred to as the effective focal length, since the focal length of the device will depend on the one or more optical elements, e.g., lenses, which make up the device and their interaction. To take good quality sharp pictures requires larger aperture lens elements (larger diameter lenses) when the effective focal length is large.
The use of large focal length lenses and optical chains with large focal lengths is often desirable because of the amount of zoom (magnification) that they can provide. However, the use of optical components which are commonly used to produce an optical chain with a large focal length tends to lead to a thick camera particularly in the case where the optical components are arranged so that the light along the optical axis passes in a straight line from the outermost lens to the sensor which is used to capture an image based on the light passing through a lens.
From the above discussion is should be appreciated that there is a need for methods and apparatus which allow a camera to use one or more optical chains or elements, e.g., with large focal lengths, but with the camera still having a relatively thin format.